The present invention relates to a driving device for a piezoelectric element, and more particularly to such a driving device capable of attaining lower power consumption, low heat generation, and low cost.
A combination of a piezoelectric element and a coil resonate at a given frequency while repeating charging and discharging. When the piezoelectric element is used for driving a print wire of a printer, it is required that a displaced condition of the piezoelectric element be maintained for a predetermined period of time. Japanese Laid Open Patent Publication No. 63-130357 discloses a driving device to this effect.
The driving device disclosed therein is arranged as shown in FIG. 1. A piezoelectric element 102 is connected through the emitter-collector path of a switching transistor 103 to the power supply terminal of a power supply 101. The switching transistor 103 serves to apply the power supply voltage to the piezoelectric element 102 in response to a driving signal V2. A diode 110 is connected in parallel across the piezoelectric element 102 so that a voltage is applied to the diode 110 in reverse direction when the switching transistor 103 is ON. Further, the non-grounded terminal of the piezoelectric element 102 is connected to one terminal of a coil 105 and another terminal of the coil 105 is in turn connected through a diode 109 to the power supply terminal of the power supply 101. The diode 109 is connected to flow a current only in a direction from the piezoelectric element 102 to the power supply terminal.
A connection between the diode 109 and the coil 105 is connected to the collector of the switching transistor 106. The switching transistor has an emitter connected to ground and a base connected to the collector of a switching transistor 108. The switching transistor 108 has an emitter connected through a resistor to the non-grounded terminal of the piezoelectric element 102, and a base connected to the collector of a switching transistor 107. The switching transistor 107 has an emitter connected to ground and a base applied with a release signal V3.
In the circuit configured as above, when the drive signal V2 is high, the switching transistor 103 is rendered conductive. Then, electric charges supplied from the power supply terminal are charged in the piezoelectric element 102 through the switching transistor 103, thereby resulting in a displacement of the piezoelectric element 102.
To restore the piezoelectric element 102, the drive signal V2 is rendered low and the release signal V3 is rendered high. Then, further supply of the electric charges to the piezoelectric element 102 is interrupted and the switching transistor 107 is rendered conductive. The base voltage of the switching transistor 108 is therefore at 0 volt. However, the emitter of the switching transistor 108 is applied with a voltage because it is connected to the piezoelectric element which has been electrically charged. Due to a voltage developed across the base and emitter of the switching transistor 108, the switching transistor 108 is rendered conductive. The base of the switching transistor 106 is therefore applied with a voltage approximately equal to the terminal voltage of the piezoelectric element 102, thereby causing to render the switching transistor 106 conductive. A series connection of the piezoelectric element 102 (which is equivalent to a capacitor) and the coil 105 forms a resonant circuit. The current flowing in the coil 105 and the voltage thereacross are in the form of a sinuasoidal waveform whose frequency is determined by the inductance of the coil 105 and the capacitance of the piezoeletric element 102. A phase difference between the current and the voltage is 90 degrees if pure resistor components included in the circuit are neglected. Further, there is no loss in the circuit due to the fact that no substantial pure resistor components are included in the circuit. If a time corresponding to a quarter of one period has been expired after the release signal is raised high, the voltage applied to the coil is zeroed as the voltage applied thereto changes in the sinusoidal waveform. At this moment, since the emitter of the switching transistor 108 is at 0 volt, the switching transistor 108 is rendered OFF and concurrently the switching transistor 106 is also rendered OFF. As a result, the current flowing in the coil 105 is entirely flowed into the power supply 101 through the diode 109. As such, the electric charges discharged from the piezoelectric element 102 are not thermally wasted, and the low heat generation and low power supply consumption are attained.
A load of relatively large mass is generally connected to such piezoelectric element. The piezoelectric element is typically urged by a resilient member such as a leaf spring in a direction to restore the displacement. Further, since the piezoelectric element is weak against a tensile force, there has been proposed a structure in which a movable member is mechanically disconnected from the piezoelectric element when the piezoelectric element is made free from the excitation. When the piezoelectric element is made free from the excitation, the movable member moves back by the aid of the leaf spring with a small delay from the time when the piezoelectric element is restored. At this time, the movable element impinges upon the piezoelectric element, whereby electric charges are produced in the electrode of the piezoelectric element.
However, with the use of the driving device as constructed above, the piezoelectric element 102 is electrically isolated because the switching transistor 106 is OFF, and thus the electric charges are held in the piezoelectric element 102. Accordingly, a small amount of displacement occurs in the piezoelectric element due to the electric charges held therein. If the piezoelectric element is used as the print wire driving mechanism of a printer, the print wire is held in a slightly projected state. This causes to bother the feeding of an ink ribbon.
Further, the conventional driving device as shown in FIG. 1 is complicated in configuration and the provision of the device at low cost is difficult as many switching transistors are used therein. Further, a loss results from the use of many switching transistors and therefore the conventional driving device is not satisfactory in terms of low power consumption.